The present invention relates generally to the field of depositing uniform layers of materials on a substrate via screen printing techniques, and more particularly to the use of an improved method of depositing a uniform layer of material on a substrate which results in the production of an improved capacitive pressure transducer.
Capacitive pressure transducers are known and generally comprise parallel plate capacitors separated by an air gap wherein the spacing between the parallel plates of the capacitors is altered in response to a sensed pressure. Generally, one electrode plate of the capacitor is deposited on a thick base substrate and the other capacitor electrode is deposited on a thin pressure sensing diaphragm which is spaced away from the base electrode by a ring of insulating glass around the periphery of the diaphragm and base substrate. Typically, the diaphragm, glass ring and base substrate are heated after assembly to form an integral assembly with the capacitor electrodes being spaced apart by a distance totally dependent upon the thickness of the glass layer. A vacuum entrance hole is provided through the base substrate and through this entryway a vacuum is applied to the air cavity bounded by the diaphragm, the glass insulating layer, and the base substrate. By applying varying degrees of vacuum through the vacuum entrance, or by maintaining a constant reference vacuum pressure in the cavity and applying various pressures exterior to the cavity, the transducer diaphragm is flexed and results in changing the capacitance created by the electrodes in response thereto. Thus by monitoring the change in capacitance, the transducer produces an electrical signal related to changes in vacuum pressure. Such transducers are readily adaptable for sensing the vacuum pressures generated by automobile internal combustion engines.
Typically, for mass producing capacitive transducers such as those described above, it is imperative to control the thickness dimension of the glass insulating ring to assure the uniform production of capacitive pressure transducers. Generally, thick film screen printing techniques using either an etched metal mask or a stainless steel such thick film screening techniques, while producing a glass insulating ring at an extremely low cost but have not been able to sufficiently produce a uniformly thick layer of glass insulating material. This is because control over the thickness of a material deposited through a pattern bearing mask primarily depends upon the thickness dimensions of the mask and these dimensions are not sufficiently controllable over the entire pattern to be deposited. In addition, generally thick film screen printing processes have been unable to produce a relatively thick (0.0016 to 0.002 inch) layer of screen printed material. This is because for depositing such a thick layer of material, which is the thickness desired for a capacitive pressure transducer insulating ring, even less control is obtainable as to the uniformity of each layer deposited by the standard thick film screening process. This is because in printing such thick layers of material, either a much thicker layer of emulsion must be used with a stainless steel screen or a much thicker plate material must be used for developing an etched metal mask. In both of these situations, less control over the uniformity of the screen or mask is obtainable and this leads to a less uniform deposit of material on a substrate.